There has long been interest in making hydrogen and oxygen from water, and numerous processes for doing so have been developed.
Recently, there has been interest in hydrogen as a medium for storage of energy and as an energy source. Thus, processes have been investigated whereby the heat supplied by power plants, by nuclear fission, nuclear fusion, or concentrated solar energy, can be utilized to split water and thereby produce hydrogen. At the same time, processes requiring hydrogen for making synthetic fuels and systems, such as fuel cells, using hydrogen to produce energy efficiently, have been developed. See Chemical and Engineering News, Nov. 21, 1977, pp. 27-28.
The oxygen obtained when water is split to make hydrogen is, like hydrogen, a valuable commodity with numerous industrial applications.
Among the known processes for splitting water are many which involve thermochemical cycles. See, e.g., U.S. Pat. Nos. 4,089,940; 4,089,939; 4,080,436 and 3,932,599.
The production of hydrogen in the formation of a graphite intercalation compound with AlCl.sub.3 and HCl has been reported. (Leong and Forsman, Synthetic Metals 6, 61-63 (1983)). While the thermal decomposition of the resulting intercalation compound is known, the products of the decomposition have heretofore been uncertain. In particular, thermal decomposition to graphite, AlCl.sub.3 and Cl.sub.2 has not been demonstrated.
The reverse Deacon process, whereby Cl.sub.2 and H.sub.2 O are reacted to make HCl and O.sub.2, has long been known and has been used in a number of water-splitting processes which involve thermochemical cycles. See Knoche et al., J. Hydrogen Energy 2, 269 (1977).
It has not been recognized heretofore, however, that formation and decomposition of a graphite intercalation compound involving a metal halide, together with production of oxygen and hydrogen halide by reaction of water with halogen, provide a thermochemical cycle for water-splitting.